Cavity resonator



J. P. KINZER' 2,593,155

CAVITY RESONATORS April 15, 1952' 2 SHEETS-SHEET 1 Filed March 7, 1947 FIG.

FIG. 2

INVENTOR J. P. K/NZER ATTORNEY April 15, 1952 J. P. KINZER CAVITY RESONATORS 2 SHEETS--SHEET 2 Filed March 7, 1947 FIG. 5

INVENTOR J. R K/NZER A TTORNE Y Patented Apr. 15, 1952 Telephone: Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application March '7, 1947, Serial No. 733,224

invention relates to cavity resonators and more? particularly to the suppression of undesiredmodes of oscillation in such resonators.-

An'object of the invention is to achieve high Q i'i'i-a cavity resonator by discriminating against unwanted modes of oscillation without concomitantl'y affecting the frequency or field pattern of the'desired mode.

Another object of the invention is to degrade unwanted modes without affecting appreciably thedesired' mode in a high Q resonator;

Another object of the invention is to split aplrClaim. (01. 178-44) preci'abl'y the frequencies of degenerate modes in. a. cavity" resonator:

Still another object of the invention is to relegaterundesi'red modes to frequencies outside the operating range without concomitantly at fecting the frequency of the desired mode.

In; accordance with an embodiment of the, invention a thin metallic fin or septum is introduced intoa cavity resonator in a manner such that it is everywhere perpendicular to the E lines" of one" of the" normal desired modes, for example; TEmn. Because of its position rela tivc to thefield patterns of other existing modes, such a finwill' tend. to absorb. theselatter; extraneous modes; and will perturb their frequencies: In. highly symmetrical cavities, it will? split" extraneous modes, which are degencrate; without concomitantly aifecting the frequency or electric: field pattern of the desired mode; 'l'heconformation. of such fins. may be annular; crescent; diametra1,. longitudinal, radial, depending on. which mode is selected as a desired-mode: and their penetration into the cavity may be partial or complete- In practice, it may, be found desirable to extendthe fin from thewall into the cavity space to a compromisejpenetration for establishing maximum absorption. of extraneous modes consistent with minimum effect on the desired mode.

Fig. 1 shows a resonant cavity operatingin a TEOln mode in accordance with the invention.

Fig. 2 is a sectional view of the cavity of Fig; 1'.

Fig. 3 shows a modification operating in a Thine-mode. I

Fig. 4' is" an explanatory diagram for Fig. 3.

Fig: 5 isa. modification showing the mode suppressing" fins on the moving piston of the cavity.

Fig. 6. is a sectional view of the cavity; shown in:Fig.; 5. 7

Referring to Fig. 1, there is shown in longitudinal section a cavity resonator I in the form of: a cylinder. having a closed, lower end 2 and a cover plate 2..

The cavity resonator and cover plate 2 may consist of electrically conductingmaterial such of the end plate 2.

as; aluminum. or copper, preferably with'an. in'- terior coating or deposit of highly conductive material; such assilver'or gold 3. 7 r

The resonator may be provided with a single feed as shown at 9 in Figs/1 and 2' or with the input'jand output connections, disclosedin' the United States application of H. B. Brehm and W. F. Kannenberg, Serial No. 687,549; filedAu gust 1, 1946 nowUnited States Patent Number 2,527,619, issued October 31, 1950. In either feed, the coupling loops pass through the end plate 2', and are tangential-to a circle in the re"- 'gion of strong electric field for oscillation of the mom mode. The loops are located" preferably half-way between the center and circumference A tuning piston 4' supported by a plunger'5 passing through a guiding sleeve 20' is moved in the interior space to tune the cavity resonator l, whereby it may have a natural resonance frequency at a normal TEo1n-desired mode, within a predetermined tuning range. A peripheralgap or clearance existsbetween the piston Sland the side wall of the cavity for suppressing TM modes as disclosed in the United States application of I. G. Wilson Serial No. 593,508, filedMay 12, 1945.

Oscillations of the desired mode 'IEom will have an electric fieldpattem of concentric lines of force with respect tothecylindrical sidewall of the resonator I. Oscillations-of extraneous and undesired modes, such as TEnn. 'IEzm, etc. may be concomitantly excited with theircharacterlstic electric field patterns. In order to effectively suppress these extraneous modes," a thin metallic fin or fins 6 is longitudinally disposed on the periphery of the cylinder Ito-extend radially inward into the cavity spaceasi1"- lustrated-in Fig; 2. Since" the fins'G are everywhere perpendicular to the electric field lines of the desired mode 'IEom, such mode will be undisturbed in its field pattern and frequency. However, other modes will be strongly suppressed to a greater or lesser extent'by the fin 6, and their field patterns disturbed.

Except for isolated cases, all the-other modes of the original cylinder I willbe perturbed in frequency since the. fields-in the empty cylinder noj'longer satisfythe new boundary conditions ofthe altered surface with fins. Furthermore, if the original cylinder was circular; its. in-

herent double degeneracy will be lost.

3 into the interior, it was determined that the change in Q was as indicated below:

Desired d TEN. D1ameter 4 is a convenient parameter, involving wavelength A, and skin depth 6 as disclosed in the United States application of J. P. Kinzer, Serial No. 588,201, filed April 13, 1945, now Patent No. 2,541,925, issued February 13, 1951.

. Thus, for example, a 20 per cent penetration of the longitudinal fin 6 involves only a 3.5 per cent degradation in Q for the desired TEom mode. As the above table indicates, narrow fins are preferable.

The extraneous and undesired modes will be concomitantly affected by a single fin 6. Thus, all modes TETmn with -r 0 will be split to some extent into two modes of different frequencies.

For example, the TE1,2,n mode will be replaced by.

two new modes. One thereof will be same frequency as the original, with field orientation such that the electric force (E) lines are perpendicular to the fin. There will be a second mode resultin from the splitting, which will be oriented generally 90 degrees from the first, and whose E lines will be badly distorted in the vicinity of the fin, and its frequency thereby lowered.

When two fins are located circumferentially 90 degrees apart in the cavity, both new modes resulting from the splitting of the TElZn mode are perturbed in frequency from the original value. There will be no orientation of the original TE12n mode which will satisfy the new boundary conditions introduced by the two fins. If both fins are identical, the frequency perturbations would be equal and a double degeneracy would ensue.

' In some applications, where maximum Q is not essential, larger fins may be used, for example, in a wave guide feeding a resonant cavity where it is-desired to establish only a TEOn-r wave in the guide.

Here the fins may extend virtually to the center of the guide and x of them may be present with uniform angular spacing. In this instance,

all modes TE'rmn and types of wave transmission having previously in Fig. 1 for the TEOln mode, it would be expedient to insert crescent-like fins in the interior of the cylinder, as illustrated in Figs. 3 and 4. Such fins would be arranged as shown at 1, being attached to the end plate at its periphery and extending longitudinally into the cavity space. The feed loop l9 would be centrally located on the end plate and extend along a longitudinal plane of the cavity. However, it

4 may for some purposes be desirable to develop the fin into a surface extending the entire length of the cavity. In this event, the resonant cavity would still be cylindrical but no longer circular. The tuning piston must be correspondingly shaped. As shown in Fig. 4, the electric field pattem of the operating TElln mode will everywhere be perpendicular to the crescent fin or to the crescent surface aforementioned. With the fin or superficies 1 crescent-like in cross-section present, the field pattern and frequency of the operating IEun mode for the resulting cavity resonator will be the same as for the original cylinder I. However, all other extraneous modes (includingthe companion or degenerate TElln mode whose E lines were originally everywhere perpendicular to the operating T'Elln mode) are absorbed to a considerable extent, and their fre- :quencies perturbed by the new interior surface of the resonator, due to the newly established boundary conditions for their electric field patterns.-

Referring to Fig. 5, the cavity resonator or ring box i is provided with diametral metallic fins 8-8 fastened to the moving, tuning piston 4 and perpendicular to the face thereof. The fins 88 are arranged to be perpendicular to the electric force lines of the main TEOln mode and parallel to the E lines of the TE122 mode, for which it will introduce large loss. A single coupling loop 9 is shown, for introducing the microwave energy into the cavity. It should be understood that two loops may be used in the manner disclosed in the aforementioned United States application of H.

B. Brehm et al., Serial No. 687,549, filed August 1, 1946, now Patent No. 2,527,619, issued October 31, 1950. In the latter case, the fins 8-8 are disposed so as to be perpendicularto the line joining the coupling loops.

What is claimed is: In combination, a tunable cavity resonator of high Q having a cylindrical side wall and parallel end walls, the upper end wall being a movable reflecting piston formin a peripheralgap with said side wall, a conductive coupling loop passing through the other end wall for exciting a TEoin mode at a position of high field intensity therefor, a plurality of thin, equiarigularly spaced conducting fins connected longitudinally/"to the side wall, said fins extending radially into said resonator to split degenerate modes and shift the frequency of extraneous TE'rmn modes, where 1- 0 to frequencies outside the tuning range of saidresonator. w JOHN P. KINZER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,129,669 Bowen Sept. 13, 1938 2,129,714 Southworth Sept. 13, 1938 2,180,950 Bowen Nov. 21, 1939 2,199,083 Schelkunofi Apr. 30, 1940 2,426,177 Carlson Aug. 26, 1947 2,439,388 Hansen Apr. 13, 1948 2,458,579 Feldman Jan. 11, 1949 2,471,419 Edson et al May 31, 1949 2,476,034 Fox July 12, 1949 2,496,772 Bradley Feb.'7, 1950 2,501,545 Sproull Mar. 21','1950 2,513,334 Kirkman July 4, 1950 2,527,619 Brehm Oct. 31, 1950 

